Vehicles, and in particular off-highway machinery such as wheel loaders, excavators, dozers, tractors, harvesters and similar heavy duty machines often operate with hydraulic steering systems. Typically, the steering system receives a desired steering input to indicate a desired angular movement of the vehicle. The steering input could be provided by an operator via a steering wheel. In a fluid controller, the steering input is converted into a hydraulic flow to a hydraulic actuator, e.g. a hydraulic cylinder which moves the steering wheels of the vehicle. A fluid controller of the above mentioned kind is disclosed e.g. in U.S. Pat. No. 5,992,458 and in U.S. Pat. No. 5,638,864 wherein the controller is disclosed in combination with an unequal area steering cylinder.
In fluid controllers of the mentioned kind, the valve member generally has a neutral configuration and two operating configurations on opposite sides of neutral, corresponding to a left-turn and a right turn of the vehicle. When the valve member is moved relative to the housing, fluid is initially pumped manually towards the actuator. Due to resistance in the fluid meter, the movement of the valve member relative to the housing also results in movement of the valve member from the neutral configuration to one of the operating configurations whereby a fluid path is opened from the inlet port and thereby from a high pressure pump towards one of the control ports and thereby to the steering cylinder, and from the other control port towards the outlet. In that way, a hydraulic piston may be displaced in a cylinder and the steered element of the vehicle is moved. The fluid controllers can be divided into two essentially different groups, namely a reaction type and a non-reaction type controller. In the reaction type controllers, a closed loop is established when the valve member is in the neutral configuration. In the closed loop, the fluid may bypass the source of pressurised fluid, i.e. bypass the hydraulic pump which creates the steering pressure and also bypass the system reservoir. In the loop, the fluid flows directly from a head end of the cylinder through the controller and towards a rod end of the cylinder, and in practise, this allows the steering wheel to follow the movement of the wheels. In the non-reaction type controller, the passage between the head end and the rod end of the cylinder is closed when the valve member is in the neutral configuration. During normal use of the vehicle, the reaction type controller enables the operator to sense resistance and hindrance on the road and allows movement of the vehicles steering system with no interaction from the operator and therefore facilitates towing of the vehicle.
To control the extent of the displacement, the controllers normally include a fluid meter with chambers which, during rotation of the valve member, expands and contracts to meter the fluid which flows from the inlet towards the control port relative to the extent of rotation of the valve member. The chambers communicate with the valve member via a plurality of meter ports located in an outer surface of the valve member and via corresponding separate openings provided in the housing into each of the chambers. The openings are located in the housing to communicate individually with the meter ports during rotation of the valve member in the housing. In addition to the metering of the fluid, the fluid meter normally facilitates the abovementioned manual pumping of the fluid until the valve member moves from the neutral configuration to the operating configuration and the high pressure pump thereby takes over the movement of the steering cylinder.
The fluid meter comprises a fixed number of chambers, typically seven chambers, and the valve member comprises a larger amount of meter ports which alternatingly, during rotation of the valve member, communicate with individual chambers. Typically, the valve members of existing controllers comprise 12 such ports. In the neutral configuration of the valve member, all 12 meter ports are in no communication with the inlet. When the valve member is moved from the neutral configuration towards one of the operating configurations, a fluid path is opened internally in the valve member from an inlet of the valve member towards every second meter port, and from every other meter port towards a cylinder port which communicate with a corresponding control port of the housing. As the valve member rotates in the housing, each chamber is, by turns, connected to the inlet and to the control port.
Some of the ports communicate with expanding chambers, some of the ports communicate with contracting chambers, and some of the ports are not in communication with the fluid meter. As a result, an uneven pressure may exist in the meter ports, and it has been found that this may result in an increased resistance against rotation of the valve member in the housing.